Sandwich panel with a ductile hybrid core comprising tubular reinforcements

ABSTRACT

A hybrid core comprising a matrix and tubular structural reinforcements, such as a ductile matrix core reinforced with ductile hollow tubes, as well as panels which comprise such hybrid core. Embodiments of the invention manifest resistance to both interfacial debonding and ballistic penetration.

This is the United States national stage of international applicationPCT/US2013/068895, international filing date Nov. 7, 2013, which claimsthe benefit of the Nov. 8, 2012 filing date of U.S. provisional patentapplication Ser. No. 61/723,844 under 35 U.S.C. §119(e). The completedisclosure of the priority application is hereby incorporated byreference in its entirety.

This invention was made with government support under grant NNX07AT67Aawarded by the National Aeronautics and Space Administration; grantNNX11AM17A awarded by the National Aeronautics and Space Administration;and, supported by the NSF under grant number CMMI0900064. The UnitedStates government has certain rights in the invention.

TECHNICAL FIELD

This invention relates to structural materials, more particularly itrelates to sandwich panels comprising hybrid core comprising a ductilematrix which is reinforced by tubular structural elements.

BACKGROUND ART

Advanced composite sandwich structures have been widely used inaerospace structures, autos, armors, wind turbine blades, pipelines,bridge decks, etc. due to their superior structural capacity in carryingtransverse loads with minimal weight penalty [1-4]. Although sandwichconstruction has been extensively used in various fields, sandwichpanels have not been fully exploited in critical structural applicationsdue to damage tolerance and safety concerns.

Sandwich structures typically consist of skins (surfacing and backplates) and a core. The skins are mainly responsible for carrying thebending moment (in blast protective sandwich panels, the surfacing plateis also responsible for eroding, breaking and slowing down theprojectiles) and the core takes care of separating and fixing the skin,carrying the transverse shear load, providing impact resistance, andtaking other functional duties. Numerous efforts have been made toexplore high-performance sandwich panels over the past several decades.

For a typical sandwich structure, three elements dominate itsperformance and function: the face sheets, the core, and the bondbetween the core and the face sheet. A major problem of sandwich panelsis the debonding at or near the core/face sheet interface, especiallyunder impact loading, which can lead to a sudden loss of structuralintegrity and cause catastrophic consequences. Such debonding may alsorestrict the contribution of impact energy absorption by the core to theentire sandwich structure. Despite the efforts in previous studies, thisproblem has not been well addressed.

Various types of core materials have been studied such as foam core(polymeric foam, metallic foam, ceramic foam, balsa wood, syntacticfoam, etc.) [3, 4], truss, honeycomb and other web cores [5], 3-Dintegrated core [6, 7], foam filled web core [7, 8], laminated compositereinforced core [9], etc. While these core materials have been used witha certain success, they are limited in one way or another. For example,the brittle syntactic foam core absorbs impact energy primarily throughmacro length-scale damage, sacrificing residual strength significantly[10-12]; and web cores often lack suitable bonding with the skin andalso have impact windows [7, 8]. An impact window is the open space in acore, which allows easy perforation of projectiles or escape of anything(e.g., fluid) which may be contained behind the panel. Among the foamcores, metallic foam has also been developed. Metallic foam material hasreceived rapid and intensive attention over the past decade due to itshigh specific stiffness and superior energy absorption ability [13, 14,15].

Previously, it was found that by filling the empty bays formed bycontinuous fiber reinforced polymer grid skeleton with polymericmaterial, the resulting composite sandwich could be improved as toimpact mitigation [16, 17, 18, 19], although these approaches continuedto have important limitations.

Prior approaches did address impact mitigation by:

(1) each cell is a small panel or mini-structure with elastic boundary,it thus tends to respond to impact in a quasi-static manner, i.e.,similar to the behavior under static load;

(2) the periodic grid skeleton, the primary load carrying component with2-D continuity, could be responsible for transferring the impact energyelastically, dissipating the energy primarily through vibration dampingand providing the in-plane tensile strength and in-plane shearresistance;

(3) the light-weight polymer matrix in the bay, the secondary loadcarrying component, could be primarily responsible for absorbing impactenergy through damage;

(4) the grid skeleton and the polymer in the bay could develop apositive composite action, i.e., the grid skeleton confines thepolymeric bay to increase its strength and the polymer matrix provideslateral support to resist rib local buckling and crippling. In addition,the polymeric bay could also provide additional in-plane shear strengthfor bi-grids such as orthogrid; and

(5) the core and skin could be fully bonded because the bay is fullyfilled, without the limitation of web cores.

However, with the prior approaches it is found that when the impact ison the rib or node of the grid skeleton, the residual strength isreduced considerably, due to the brittleness of the glass fibers [see,e.g., 17, 19]. For example, with about 300 J of impact energy, theprojectile perforated the panel, suggesting poor perforation resistance[19]. Furthermore, because the impact caused fracture of the reinforcingfibers [19], and because the fibers are the primary load carryingcomponent, this caused prior sandwich panels to lose their load carryingcapacity permanently. Consequently such panels were radically impairedwith subsequent impacts such as may occur in attack or militarysituations. Additionally, as the projectile impacted these materials itbroke the reinforcing fibers, consequently the impact energy or impactwave could not be distributed or absorbed by the whole structure andthis led to local perforation.

Another major problem of sandwich panels prior to the present inventionhas been debonding at or near the core/face sheet interface under impactloading, which can lead to a sudden loss of structural integrity andcause catastrophic consequences. The debonding may also restrict thecontribution toward impact energy absorption by the core to the entiresandwich structure. This problem of debonding at or near the core/facesheet interface has remained an unmet need in the field.

DISCLOSURE OF THE INVENTION

The present invention comprises sandwich panels which comprise a ductilereinforced hybrid core. The reinforcements can be metal, alloy, shapememory alloy, ceramic, composites, polymer or polymeric. Thereinforcements are in a form of millitubes/microtubes. Advantages of theinvention with regard to use in critical structural applications includebeing amenable for mass production, the design is simple forimplementation, and it possesses various favorable characteristics,preferably the panels are strong, stiff, ductile, tough, lightweight,impact-tolerant, and debonding-tolerant.

In a sandwich panel of the invention, the face sheet can be anythingunderstood by those of ordinary skill to be used or usable in practiceas face sheets. Although, a laminated composite was used in the examplesherein, it is understood that other materials or other forms such asmetal, polymer, ceramic, or hybrid composite may be comprised by theinvention as a face sheet. For instance the face sheets of sandwichpanels can be made of composite plate such as laminated carbon fiberreinforced polymer, ceramic chips, carbon nanotube enhanced FRP (fiberreinforced polymer) plate, etc.

The distribution pattern of these tubular reinforcements were parallelwith and/or perpendicular to the plane of the surface face sheets/skinplates; in certain embodiments millitubes parallel with the surfaceplane are configured as a grid. Configurations with tubes which areparallel with the plane of the skin plates (whether the tubes are inunidirectional or in various grid or woven configurations) arepreferred.

When measured at room temperature, the matrix can be, e.g., polymer,shape memory polymer, metal, glass or ceramic. Regardless of thematerial, the matrix is ductile having elongation at break in a range ofabout (3-200%); and preferably having properties of elastic modulus in arange of about 1.5-350 GPa, ultimate tensile strength in a range ofabout 25-350 MPa, and elongation at break in a range of about (3-200%).In alternative embodiments, an elastic modulus is in a range of about2-4 GPa and a yielding strength is in a range of about 30-135 MPa. Thus,these ranges relate to either a thermosetting or thermoplastic polymermatrix, e.g., a typical thermosetting polyester's ultimate tensilestrength is 34.5-130 MPa, and a typical thermoplastic polymer's ultimatetensile strength is 70-105 MPa. In one embodiment, properties of elasticmodulus, ultimate tensile strength and elongation at break of about 2.76GPa, 30 MPa and 3.5%, such as for LOCTITE Hysol 9460.

The structural reinforcements are tubular, it being understood thathollowness can achieve an advantage of weight savings, and as set forthherein have an advantage of facilitating impact energy absorption.Further, the reinforcements are ductile having elongation at break in arange of about (3-100%); and preferably an elastic modulus is in a rangeof about 1-1000 GPa and the ultimate tensile strength is in a range of1-1000 MPa, and elongation at break in a range of about 3-100%. Inalternative embodiments, an elastic modulus is in a range of about 1-210GPa and ultimate tensile strength is in a range of about 80-775 MPa.Thus, these ranges relate to either an aluminum or steel millitube,e.g., a typical aluminum millitube's ultimate tensile strength is 86-265MPa, and a typical steel millitube's ultimate tensile strength is500-760 MPa in room temperature. In one embodiment, elastic modulus,ultimate tensile strength, and elongation of aluminum tube are 14.5 GPa,121.2 MPa, and 3.01%; elastic modulus, ultimate tensile strength, andelongation of steel tube are 36.4 GPa, 527 MPa, and 65%, respectively.

Embodiments of the invention include: a hybrid core for structuralsandwich panels, the hybrid core comprising: a ductile, tubularstructural elements, and, a ductile matrix. This hybrid can comprisemultiple of the ductile, tubular structural element, and thesestructural elements are present in at least two layers; each layer ofstructural elements can be essentially unidirectional with any layer itis adjacent to; or, each layer of structural elements can be essentiallyperpendicular to any layer it is adjacent to; or, each layer ofstructural elements can be essentially aligned at an angle other than 0°or 90° to any layer it is adjacent to, such as some angle between 0° or90° such as 5°, 10°, 15°, 20°, 25°, 30°, 35°, 40°, 45°, 50°, 55°, 60°,65°, 70°, 75°, 80°, 85° or 90°. Such foregoing hybrid core can: havestructural elements of each layer are interwoven with structuralelements of any layer they are adjacent to; or, the structural elementscan comprise indentations, and the indentations in the structuralelements of a layer are interposed (or seated within) with indentationsin the structural elements of any layer of structural elements they areadjacent to. In any such hybrid core, when measured at room temperature,the ductile matrix can have: an elastic modulus within a range of1.5-350 GPa; an ultimate tensile strength within a range of 25-350 MPa;and, an elongation at break of 3-200%. In any such hybrid core theductile, tubular structural elements, when measured at room temperature,can have an elastic modulus within a range of 1-1000 GPa; an ultimatetensile strength within a range of 1-1000 MPa; and, an elongation atbreak of 3-100%.

An alternative embodiment comprises a sandwich panel comprising: ahybrid core which comprises a ductile tubular structural element; aductile matrix; and, a face sheet. In such panel the structural elementcan have a longitudinal dimension, and the element's longitudinaldimension can be parallel with a surface plane of the face sheet. In apanel the structural element can be a continuous millitube. Such panelcan comprise multiple of the structural elements; the structuralelements can be present in at least 2, 3, 4, 5, 6, 7 or more than 7layers. In such panel each layer of the structural elements can beessentially unidirectional with any layer of structural elements it isadjacent to; or, each layer of structural elements is essentiallyperpendicular to any layer of structural elements it is adjacent to; or,each layer of structural elements can be essentially aligned at an angleother than 0° or 90° to any layer of structural elements it is adjacentto such as, such as some angle between 0° or 90° such as 5°, 10°, 15°,20°, 25°, 30°, 35°, 40°, 45°, 50°, 55°, 60°, 65°, 70°, 75°, 80°, 85° or90°. In such panel, structural elements of each layer can be interwoven,and/or welded, and/or adhered or glued with structural elements of anylayer they are adjacent to. Thus, various hybrid cores were investigatedherein. As set forth in Example 1 and depicted in FIG. 1, one hybridcore consisted of polymer resin reinforced by transversely alignedcontinuous metallic millitube, denoted as “type-I sandwich panel”.Another was made of polymer resin reinforced by aligned continuousin-plane metallic millitube, denoted as “type-II sandwich panel”. Forcomparison purposes, traditional sandwich panels with polymericsyntactic foam core “type-III sandwich panel” were also prepared.

Static and impact tests demonstrated that interfacial debonding andsubsequent mixed failure by both shear and peel failure in the corecould be largely excluded from the type-II panel. Meanwhile, there was asignificant transition away from brittle failure (such as occurs withprior art) towards the more preferred ductile failure, i.e., a fracturewith a large deformation was observed in type-II sandwich panel providedpanels with dramatically enhanced load capacity and impact energydissipation.

The results indicated that type-II panel comprising layers of structuralreinforcements parallel to the sandwich panel face surface provided anappealing option for critical structural applications where the featuresof debonding resistance and impact tolerance are important.

Furthermore, having identified the surprisingly improved debondingresistance of the Type II panels further embodiments of the inventionwere developed; such as the embodiments comprising hollow metallic tubesconfigured in layers with each layer parallel to the surface of thesandwich panel, as exemplified in Examples 2, 3 and 4. Accordingly, thedata show impact tolerance of a novel sandwich with metallic hollowmillitube grid stiffened polymeric core under both low velocity impactloading (Examples 2, 3 and 4) and ballistic impact loading from gunbullets (Example 4) (In Example 1, both low velocity and hypervelocityballistic impact testing was performed.)

Accordingly, sandwich panels with reinforcements of hollow metallictubes configured in layers with each layer parallel to the surface ofthe sandwich panel were made, i.e., type-II panels as defined herein;with such embodiment the energy absorption was significantly increaseddue to the ductility of metals as defined herein (such as steel oraluminum, it being understood that non-metallics that possess ductilitysimilar to such metals are within the scope of the invention, e.g., thereinforcements can be metal, alloy, shape memory alloy, ceramic,composites, polymer or polymeric. Preferably, the reinforcements aremillitube/microtubes with a diameter of 3 mm or less, 2.5 mm or less,or, 1.5 mm or less.

Weight reduction is an objective with most structural panels, thus theuse of metal reinforcements previously has often been deemedinconsistent with the overall purpose of a sandwich material. However,by use of hollow metallic tubes, an increase in weight can be minimizedwhile at the same time producing surprisingly good debonding resistanceas well as perforation resistance.

Thus in certain embodiments, (e.g., to provide sandwich panels withdesirable and improved debonding resistance and energy absorption underimpact loadings) a hybrid sandwich core is provided wherein the corecomprises, e.g., hollow metallic millitube grid as a means to stiffenand reinforce a matrix, and where the matrix can, e.g., be a polymer oran shape memory polymer (SMP). Both the matrix and the tubularreinforcements are, respectively, ductile as defined herein.Quasi-static low velocity impact tests and ballistic impact testsdemonstrated that such new sandwich panels of the invention provide anappealing option for critical structural applications which needdebonding and multiple impact tolerance.

Definitions

“Composite” millitubes are made of composite materials. Preferably,these composite materials have ductility as set forth herein and energydissipation ability; for example, aluminum alloy, Ni—Ti Shape Memoryalloys, steel alloy or fiber reinforced polymer (FRP).

A “continuous fiber” is a type of fiber that covers the entire dimensionof a part, with few if any breaks or interruptions.

“Decomposition Temperature (T_(D))” is defined as a temperature at whichchemical bonds are broken or violent oxidation occurs to catch fire.

“Fixed strain” is the difference between the prestrain and thespringback. At the end of programming, there is a rebound or springbackwhen the load is removed.

“FRP” is an acronym for fiber reinforced polymer.

“Glass transition temperature (T_(g))”: A parameter of particularinterest in synthetic polymer manufacturing is the glass transitiontemperature (T_(g)), which describes the temperature at which amorphouspolymers undergo a transition from a rubbery, viscous amorphous liquid(T>T_(g)), to a brittle, glassy amorphous solid (T<T_(g)). Thisliquid-to-glass transition (or glass transition for short) is areversible transition. The glass transition temperature T_(g) is alwayslower than the melting temperature, T_(m), of the crystalline state ofthe material, if one exists. An amorphous solid that exhibits a glasstransition is called a glass. Supercooling a viscous liquid into theglass state is called vitrification. Despite the massive change in thephysical properties of a material through its glass transition, thetransition is not itself a phase transition; rather it is a phenomenonextending over a range of temperatures and is defined by one of severalconventions. Several definitions of T_(g) are endorsed as acceptedscientific standards. Nevertheless, all definitions are arbitrary, andthey often yield different numeric results: at best, the defined valuesof T_(g) for a given substance typically agree within a few Kelvin.

“Healing Temperature (T_(H))”: The healing temperature can be definedfunctionally as a preferred temperature above the melting temperaturewhere the thermoplastic molecules further overcome intermolecularbarriers and are able to gain mobility and to more effectively diffuse.

A “hybrid core” of a sandwich structure is one comprising a matrix aswell as structural reinforcements.

A “matrix” in a hybrid core is the material which generally surroundsand is supported by structural reinforcements; a matrix can be any of avariety of materials, e.g., polymer, metal, glass, or ceramic.

“Melting point (T_(m))”: The term melting point, when applied topolymers, is not used to suggest a solid-liquid phase transition but atransition from a solid crystalline (or semi-crystalline) phase to astill solid amorphous phase. The phenomenon is more properly called thecrystalline melting temperature. Among synthetic polymers, crystallinemelting is only discussed with regards to thermoplastics, asthermosetting polymers decompose at high temperatures rather than melt.Consequently, thermosets do not melt and thus have no T_(m).

As used herein, the terms “millitube” and “microtube” are to be seen asinterchangeable unless the context clearly indicates otherwise.Generally, millitubes are tubes with an outer diameter of at least amillimeter, and microtubes are tubes with an outer diameter of at leasta micron and up to 3 mm or less.

“Prestrain” is the maximum strain applied during programming.

“Relaxation time” is the time elapsed during stress relaxation process.

“Shape fixity” is similar to strain fixity, suggesting that a temporaryshape is fixed.

“Shape fixity ratio” is the ratio of the strain after programming overthe prestrain.

“Strain” is defined as the change in length over the original length.

“Strain recovery” is the amount of strain that is recovered during shaperecovery process.

“Stress” is defined as the internal load per unit area.

“Stress relaxation” is a phenomenon that, once a material is deformed toa certain deformation, the stress continuously reduces while maintainingthe strain constant.

“Yield strain” is the strain corresponding to yielding. In thestress-strain curve, the change of slope signals the start of yielding.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1: Typical sandwich panels with different cores: (a) type-I panelwith vertically aligned (relative to the surface deemed horizontal)hybrid core; (b) type-II panel with horizontally aligned (relative tothe surface deemed horizontal) hybrid core; and (c) type-III panel withtraditional syntactic foam.

FIG. 2: Typical compressive strain-stress curves of the three types ofsandwich panels with different cores. Type-1 panel (

); Type-2 Panel, (

) Type-3 panel, (

).

FIG. 3: Typical bending deflection-force curves of the three types ofsandwich panels with different cores. Type-1 panel, (-●-●-●-); Type-2panel (●●●●●) Type-3 Panel, (

).

FIG. 4: Impact response of different types of sandwich panels withdifferent cores: (a) type-I panel; (b) type-II panel; and (c) type-IIIpanel

FIG. 5: Experimental observation of type-II panel after impact loading:(panel a) shows intact micro-scale local skin/core bonding; (panel b)minor debonding at millitube/matrix interface; (panel c) globaldeformation; (panel d) excellent bonding at millitube/matrix interface

FIG. 6: Impact failure mode of panels with hybrid core: (a) type-Ipanel; and (b) type-II panel

FIG. 7: Schematic of novel microtube grid stiffened foam core. Panel a:a millitube showing cross-sections at A-A and B-B, with B-B being one ofthe periodic indentions. Panel b: millitubes aligned in longitudinal andtransverse directions to form one layer of grid skeleton; with a node, abay and a rib labeled. A node occurs when two perpendicular millitubesintersect, in each case at the point of the respective periodicindentations. Panel c: perspective view with more layers ofperpendicular millitubes added. Panel d: another perspective view of amultiple layer core shown in panel c; Panel e: depiction of a multiplelayer core after foam (light grey shading) was poured into the gridskeleton and then cured.

FIG. 8: Schematic of two longitudinal tubes (tube 1 and tube 2)intersecting with one transverse tube (tube 3) at the pre-indentedlocation thereby creating a “node”. The presence of optional adhesive isdepicted by the solid black areas between the various tubes. Anotheroptional approach to assembling tubes is to weld them at the “node”area.

FIG. 9: Impact response of sandwich panels set forth in Example 2.

FIG. 10: Impact response of a type II sandwich panel which comprises ametallic microtube-reinforced shape memory polymer matrix, as set forthin Example 3.

FIG. 11: Ballistic impact testing setup (not to scale) showing a gun 1meter from a target, a projectile, and the location of the laserspeedometer.

FIG. 12: Typical compressive strain-stress curves of the three types ofsandwich panels (each with a different core). G-1 panel, ( - - - - );G-2 panel (

) G-3 Panel, (

).

FIG. 13: Typical bending stress-strain curves of the three types ofsandwich cores (each with a different core). G-1 Panel, (

) G-2 panel, (

); G-3 panel (

).

FIG. 14: The typical load-time and energy-time responses of the G-3sandwich panels at three locations: (panel a) bay area, (panel b) nodearea, and (panel c) rib area. In each panel: Energy ( - - - - ); Load (

).

FIG. 15: The maximum load and propagation energy for different impactlocations on the panels: (panel a) maximum load, (panel b) propagationenergy. In each panel data is shown by: white bars for bay; black barsfor node; crosshatch bars for rib.

FIG. 16: Failure mode of G-1 sandwich panel after 9 mm bullet ballisticimpact test: (panel a) top view of FRP face sheet with an oval (0)indicating the impact area; (panel b) bottom view after peeling off FRPback sheet showing a deformed bullet (1).

FIG. 17: Failure mode of G-2 sandwich panel after ballistic impact test:(a) top view of sandwich core showing a deformed bullet (2); (b) bottomview of sandwich core with an oval (3) indicating the impact effectarea. In these views both the face sheet and back sheet were pulled offin order to view the damaged core.

FIG. 18: Failure mode of G2 sandwich panel after 0.22 bullet ballisticimpact test with the location of the bullet (4) indicated.

FIG. 19: Failure mode of G2 sandwich panel after double ballistic impacttesting including impact by a .22 caliber bullet and a 9 mm bullet: (a)top view with location of .22 caliber bullet (5) and a 9 mm bullet (6);(b) bottom view, with the respective impact areas designated by ovals.22 caliber bullet (7) and 9 mm bullet (8).

MODES FOR CARRYING OUT THE INVENTION

Sandwich construction has been extensively used in various fields.However, sandwich panels have not been fully exploited in criticalstructural applications due to the concern of debonding and impactdamage. To address these problems, the present invention sets forth anew hybrid core based sandwich panels; such hybrid cores comprise aductile matrix and ductile tubular reinforcements.

When measured at room temperature, the matrix can be, e.g., polymer,shape memory polymer, metal, glass or ceramic. Regardless of thematerial, the matrix is ductile, having elongation at break in a rangeof about (3-200%); and preferably having properties of elastic modulusin a range of about 1.5-350 GPa, ultimate tensile strength in a range ofabout 25-350 MPa and elongation at break in a range of about (3-200%).In alternative embodiments, an elastic modulus is in a range of about2-4 GPa and a yielding strength is in a range of about 30-135 MPa. Thus,these ranges relate to either a thermosetting or thermoplastic polymermatrix, e.g., a typical thermosetting polyester's ultimate tensilestrength is 34.5-130 MPa, and a typical thermoplastic polymer's ultimatetensile strength is 70-105 MPa. In one embodiment, properties of elasticmodulus, ultimate tensile strength and elongation at break of about 2.76GPa, 30 MPa and 3.5%, such as for LOCTITE Hysol 9460.

The structural reinforcements are tubular, it being understood thathollowness can achieve an advantage of weight savings, and as set forthherein have an advantage of facilitating impact energy absorption.Further, the reinforcements are ductile having elongation at break in arange of about (3-100%); and preferably an elastic modulus is in a rangeof about 1-1000 GPa and the ultimate tensile strength is in a range of1-1000 MPa, and elongation at break in a range of about 3-100%. Inalternative embodiments, an elastic modulus is in a range of about 1-210GPa and ultimate tensile strength is in a range of about 80-775 MPa.Thus, these ranges relate to either an aluminum or steel millitube,e.g., a typical aluminum millitube's ultimate tensile strength is 86-265MPa, and a typical steel millitube's ultimate tensile strength is500-760 MPa in room temperature. In one embodiment, elastic modulus,ultimate tensile strength, and elongation of aluminum tube are 14.5 GPa,121.2 MPa, and 3.01%; elastic modulus, ultimate tensile strength, andelongation of steel tube are 36.4 GPa, 527 MPa, and 65%, respectively.

As set forth in Example 1 below, results demonstrated for example that:(i) interfacial debonding at or near the face sheet/core were largelyexcluded from type-II panels with horizontally aligned millitubes; (ii)as contrasted with the brittle failure that occurred in the traditionalsyntactic foam cored sandwich panels, significant ductile failure wasachieved in type-II panels of the invention; and (iii) the compressivestrength, flexural strength, and impact response of type-II panel withthe hybrid core showed dramatic enhancements over the traditionalsyntactic foam-cored sandwich panel.

This work indicated that type-II panels are an option for criticalstructural applications requiring debonding resistance and/or impacttolerance.

Moreover, as set forth in Example 2 disadvantages of prior artcontinuous fiber reinforced grid skeletons that contain glass fiberswere overcome by embodiments of the invention that comprised a hybridcore reinforced with ductile structural units or elements aligned in agrid configuration. These embodiments retained the favorable debondingcharacteristics of the type-II panels of Example 1, while concomitantlyexhibiting substantially improved impact resistance characteristics.Accordingly, ductile reinforcements comprising hollow metallic tubesconfigured in non-unidirectional layers such that each layer isessentially parallel to the surface of the sandwich panel yet eachadjacent layer is not directed in the same direction as its neighbor;typically the adjacent layers were aligned in a direction 90° shiftedfrom its adjacent neighbor, creating a grid. Advantageously, with such“grid” embodiments the energy absorption was significantly increased aswas impact resistance.

EXAMPLES Example 1: Comparison of Sandwich Panels with Either in-Planeor Out-of-Plane Metal Tube Reinforced Foam Core

In this example, two types of hybrid cores and one traditional syntacticfoam core were evaluated. The first hybrid core consisted of vertically(transverse relative to the surface plane) aligned metallic millitubesand polymer resin, which was denoted by type-I core (the correspondingsandwich panel was denoted by type-I panel). The type-I core isillustrated in FIG. 1a . The second hybrid core consisted ofhorizontally (in-plane with that of the surface) aligned metallicmillitubes and polymer resin which was denoted by type-II core (thecorresponding sandwich panel was denoted by type-II panel). The type-IIcore is shown in FIG. 1b . A traditional syntactic foam core (glassmicroballoon based) was selected as the reference, denoted as type-IIIcore (the corresponding sandwich was denoted by type-III panel). Thetype-III core is plotted in FIG. 1 c.

Materials

The face sheets of all three types of sandwich panels were made oflaminated composite plate. The composite plate was prefabricated bybi-directional woven glass fabric reinforced vinyl ester resin with auniform thickness of 3.2 mm (⅛ inch), (Industrial Plastic Supply, Inc.,Anaheim, Calif.). The volume fraction of the glass fiber in these facesheets was 55%. The density of the composite plate face sheet was about1.75 g/cm³. The elastic moduli of the composite plate face sheet were 18GPa, 16 GPa, and 5.5 GPa along direction one, direction two anddirection three, respectively. Direction one was aligned to the wrapdirection, direction 2 was the weft direction, and direction 3 was thetransverse direction.

The metallic millitubes for the hybrid cores (type-I and type-II cores)were made of aluminum 6061-T6. The elastic modulus and yielding strengthof the aluminum tube were 72 GPa and 120 MPa, respectively. The measuredinner diameter and outer diameter of the aluminum millitube were 1.78 mmand 2.38 mm, respectively. Thus, this particular example demonstratesthe efficacy of the invention, e.g., when the structural reinforcementis a metallic millitube. It is to be noted that in accordance with thepresent invention the particular material used as a structuralreinforcement does not matter so long as the respective reinforcementunit possesses characteristics such as those set forth herein; e.g., thereinforcements can be metal, metallic, ceramic, composites, polymer orpolymeric.

LOCTITE Hysol 9460, an epoxy based structural resin was used in allthree types of sandwich cores. (Henkel Corporation, Dusseldorf,Germany). The density of the epoxy resin was 1.31 g/cm³. According tothe manufacturer's test data, the elastic modulus, tensile strength andelongation at break were 2.76 GPa, 30 MPa and 3.5%, respectively. Thesyntactic foam core (type-III core) was made of glass microballoons andthe epoxy resin (LOCTITE Hysol 9460). The glass microballoons (EllsworthAdhesives, Germantown, Wis.) had an effective density of 0.14 g/cm³ withparticle diameter range of 5˜200 μm. The average outer diameter and wallthickness of the glass microballoons were 85 μm and 0.8 μm,respectively.

Specimen Preparation

The three types of sandwich panels were fabricated with two face sheetsand the corresponding sandwich core. The only difference between type-Iand type-II sandwich panels was the aligning pattern of the aluminummillitubes in their cores as mentioned before. In the current study, thevolume fraction of aluminum millitubes was 40% (the remaining 60% wasthe epoxy resin) for both type-I and type-II cores. Therefore, the twohybrid cores had the identical density of 1.26 g/cm3. The volumefraction of glass microballoons in the type-III core (syntactic foamcore) was also 40%, and the remaining 60% was the epoxy resin. Hence,the theoretical density of the syntactic foam core was about 0.84 g/cm³.

For all test configurations, the heights of the core and the entiresandwich panels were 12.3 mm and 18.7 mm, respectively. Therefore, thedensity (including core and face sheets) of type-I and type-II sandwichpanels was 1.41 g/cm3, and the density of type-III sandwich panel was1.13 g/cm³. It is noted that the millitube phase only contributed 37% tothe entire density of the type-I and type-II sandwich panels.

The procedure for preparing type-I and type-II hybrid cores wereidentical. The aluminum millitubes were first stacked in a mold byinserting the tubes into orifices in the side walls of the mold beforeinfusing the polymer resin. The polymer resin was then infused into themillitube preform by an optional Resin Infusion Molding (RIM) or VacuumAssistance Resin Infusion Molding (VARIM) system, depending on theviscosity of polymer [20]. In VARIM, vacuum is applied to the outlet ofa mold, and resin is drawn into the mold by vacuum only, in accordancewith methodologies known in the art (see, e.g., [20]). After curing atroom temperature, the cast cores were demolded and postcured (55° C. for4 hours). The surfaces of the hybrid core were ground and cleaned beforeapplying the skins. The prefabricated laminated face sheets(E-glass/vinyl ester) were then bonded to the hybrid cores with theidentical polymer resin (LOCTITE Hysol 9460) to form the respectivesandwich panels.

It is noted that the mass production of the hybrid cores can be easilyrealized using the existing equipment and procedures in the compositeindustry. Therefore, ease of mass production is an advantage of theinstant sandwich panels.

Geometry and Test Configuration

All three types of sandwich panels were subjected to three types oftests: (i) static compression tests; (ii) static three-point bendingtest (simply supported at two ends); and (iii) central point impact test(simply supported at two ends).

In the static compression tests, all type-I, type-II, and type-IIIsandwich panels were 25.4 mm long, 25.4 mm wide and 18.7 mm thick (allcores are 12.4 mm thick). In the static three-point bending tests, allsandwich panels were 101.6 mm long (with a test span of 90 mm), 25.4 mmwide and 18.7 mm thick (all cores were 12.4 mm thick). For the staticcompression and bending tests, strain controlled loading mode wasemployed using MTS 810 machine (MTS Systems Corporation, Eden Prairie,Minn.) with a loading rate of 1.5 mm/min.

The impact tests were conducted by the Instron Dynatup 8250 HV Impacttesting machine (Instron Industrial Products, Grove City, Pa.) with ahemi-sphere tup nose (the diameter is 12.7 mm). All type-I, type-II, andtype-III sandwich panels were 101.6 mm long, 25.4 mm wide and 18.7 mmthick (identical dimensions with the bending test specimens). Theinitiation energy and propagation energy were calculated using a dataacquisition system integrated in the impact test machine. An identicalimpact velocity of 4 m/s was used for all types of sandwich panels.However, two different hammer weights were considered: 25 kg hammer wasused for type-I and type-II panels, and 5 kg hammer was used fortype-III panels, respectively. The type-III panel was very brittle andcrack start initiation and propagation occurred by using a hammer ofonly 5 kg. When the 25 kg hammer was used the type-III panel becamecrushed, and thus there were no test results.

Results

The typical compressive strain-stress curves of the three types ofsandwich panels are plotted in FIG. 2. One can see that the type-Isandwich panel had the highest compressive strength, which was near 100MPa. The type-II sandwich panel had the intermediate compressivestrength, which was near 45 MPa. The type-III sandwich panel had thelowest compressive strength, which was only about 27 MPa.

The typical bending load-deflection curves of the three types ofsandwich panels are plotted in FIG. 3. The flexural strength of type-Iand type-II sandwich panels were about as 510% and 460% as that oftype-III sandwich panel, respectively. The two types of panels withhybrid cores (type-I and type-II panels) have comparable flexural(bending) strengths, although type-I panel has higher compressivestrength than that of type-II panel.

Pursuant to the impact testing, the typical load-time and energy-timeresponses of the three types of sandwich panels under impact loadingsare shown in FIG. 4a , FIG. 4b , and FIG. 4c , respectively. Meanwhile,the peak load, initiation energy and propagation energy of the threetypes of panels under impact loadings are listed in Table 1.

TABLE 1 Test results of the two types of sandwich panels with differentcores Maximum bending load (N) Peak Initiation Residual Types of impactenergy Propagation Before capability specimens load (KN) (J) energy (J)impact After impacts (%) Type-I 38.5 98.1 91.9 9,010 N.A. N.A. sandwichpanel Type-II 18.0 59.5 130.5 7,960 6,010 75% sandwich panel Type-III6.6 24.0 14.5 1,700 0  0% sandwich panel Specific 446.7% 326.9% 507.0%426.2% N.A. N.A. property ratio I/III Specific 218.2% 198.3% 720.1%376.5% N.A. N.A. property ratio II/III

The impact energy corresponding to the maximum impact force (i.e., thepeak force in the impact force-time curve) is defined as initiationenergy. Propagation energy is defined as the difference between themaximum impact energy and the initiation energy. These definitions havebeen used previously [22]. It has been suggested that the initiationenergy is basically a measurement of the capacity for the target totransfer energy elastically and higher initiation energy usually means ahigher load carrying capacity; on the other hand, the propagation energyrepresents the energy absorbed by the target for creating andpropagating gross damage. Generally, conventional sandwich panels withhigher propagation energy have lower residual load carrying capacitybecause the absorbed impact energy by the panels has been used to createdamage in a detrimental way, such as core/face sheet interfacialdebonding. However, a sandwich panel of the invention has higherpropagation energy, but still has higher residual load carrying capacitybecause the damaging energy has been handled in a less detrimental way,such as by matrix microcracking, rather than debonding or completematerial failure.

DISCUSSION

Specific Mechanical Properties:

Note that the results are also given in terms of specific properties inTable 1. For instance, the specific peak impact load is defined as thepeak impact load per weight of the material. Although type-I and type-IIpanels were slightly heavier than type-III panel, the specificproperties of the proposed hybrid foam based panels was significantlyhigher than that of the traditional syntactic foam based panel, whichexemplified the excellence of the hybrid cores set forth herein.

Interfacial Debonding:

A typical interfacial debonding in type-III sandwich panel (withsyntactic foam core) was observed under both static and impact loadingconditions. This interfacial debonding immediately caused the failure ofthe entire sandwich panel. For type-I sandwich panels (with verticallyaligned hybrid core), a typical interfacial debonding was observed dueto the stress concentration and the insufficient interfacial bondingarea. Although the areas are the same in all specimens, because Type IIIadded 40 vol. % microballoons, these balloons make the contact areainsufficient. However, for the Type-II sandwich panels as shown in FIG.5a , there was almost no interfacial debonding locally after the impactloadings. Only some micro-scale debondings between the millitubes andmatrix were observed as shown in FIG. 5b , while most millitube/matrixinterfacial bondings were still very good (see FIG. 5c and FIG. 5d ). Asignificant ductile deformation and failure mode were observed which is,in fact, an advantage because of the reinforcement metallic millitubes'material properties. It is believed that the horizontally aligned hollowmillitubes provide the flexibility to the sandwich panel by allowinglarge deformation and plastic deformation and squeeze within themillitubes. The Type-II sandwich panel was found to have excellentflexibility and strongly suppressed the interfacial debonding of theface sheet.

Impact Failure Mode:

When the type-I panels (with vertically aligned hybrid foam core) weresubjected to an impact loading, a large impact hole was created. Thisimpact hole caused a significant loss of face sheet and core material asshown in FIG. 6a . Meanwhile, some visible interfacial debondingsoccurred between the face sheet and core after the impact. The type-IIIpanel (with traditional syntactic foam) immediately failed after theimpact due to the interfacial debonding. However, when subjected to theimpact loadings, the type-II panels represent excellent flexibility andenergy absorption ability. The plastic deformation of the aluminummillitubes become the dominant contributor of absorbing impact energy,and most energy dissipations are assigned to the metallic millitubes.This mechanism protects the polymer matrix from brittle crazing even thematrix is quite brittle (with only about 3.5% maximum elongation). It isalso noted that the first few rows of the horizontally alignedmillitubes in the type-II panel also act as a cushioning layer toprotect the interface between the core and front face sheet. Accordingto our observation, almost no interface debonding occurred in thetype-II specimen as shown in FIG. 6b . As a particular advantage,type-II embodiments provide a solution to the interface debondingproblem which is a major concern in the traditional foam cored sandwichstructures.

Post-Impact Performance:

The post-impact performance for the critical structural members is animportant consideration. When the type-I panels (with vertically alignedhybrid foam core) were subjected to an impact loading, a large impacthole was created (see FIG. 6a ). This impact hole caused a significantloss of face sheet and core material. As a result, the residualstructural capacity of the type-I panel after impact becomes verylimited. When subjected to an impact loading, the type-III panels (withtraditional syntactic foam) immediately failed. As a result, type-IIIpanel lost all the structural capability after the impact. In contrast,after being subjected to two impact loadings, the type-II panels (withhorizontally aligned hybrid foam core) still retained 75% of the loadcapacity even though their front face sheet has been severely damaged.This superior post-impact performance of type-II embodiments is highlydesirable and it can dramatically improve the reliability of structuralmembers.

Example 2: Sandwich Panels with Metal Tube Grid as Reinforcement of aFoam Core

In this example, a type-II panel embodiment comprising a metallicmillitube grid that reinforces a polymer cored sandwich panel is setforth. The new core of this Example is a hybrid core comprising hollowmetallic millitubes in the form of a grid. This differed from thetype-II core embodiment of Example I in that the core here comprised apolymer resin reinforced by metallic millitubes which are now in a gridconfiguration rather than all millitubes in parallel as in Example 1.Low velocity impact testing demonstrated that the new grid panel is anoption for critical structural applications where features such asdebonding resistance and multiple impact tolerance are important.

It was found that by filling the empty bays in the grid skeleton withsyntactic foam (the same polymer resin as in Example 1), the resultingcomposite sandwich was an advantageous structure for impact mitigationbecause (1) each cell is a small panel or mini-structure with elasticboundaries, it thus tends to respond to impact quasi-statically; (2) theperiodic grid skeleton, the primary load carrying component with 2-Dcontinuity, is understood to be responsible for transferring the impactenergy elastically and providing the in-plane strength and transverseshear resistance; (3) the extremely light-weight syntactic foam in thebay, the secondary load carrying component, is understood to beprimarily responsible for absorbing impact energy through damage; (4)the grid skeleton and the foam together are understood to develop apositive composite action, i.e., the grid skeleton confines the foam toincrease its strength and the foam provides lateral support to resistrib local buckling and crippling. In addition, the foam is understood toprovide further in-plane shear strength for bi-grids such as orthogrids;and (5) the core and skin can be fully bonded because the bay is fullyfilled, without the limitations that come with web cores. In embodimentsof the invention, a web core is a core having connected rods with largeconnected open space; a foam core is a core with small connected space(open-celled foam) or discrete voids (syntactic foam). In other words,web core has less bonding area with the face sheet (correlating withmore debonding) while foam core has large bonding area with the faceplate.

Materials:

The face sheets of sandwich panels were conventional, and made oflaminated composite plate. (Industrial Plastic Supply, Inc., Anaheim,Calif.) The composite plate was prefabricated by bi-directional wovenglass fabric reinforced vinyl ester resin with a uniform thickness of3.2 mm (⅛ inch). Upon measuring the face sheet material it was foundthat the volume fraction of the glass fiber was 55% and 45% of thevolume fraction was the polymer.

The density of the composite plate face sheet was about 1.75 g/cm³. Theelastic moduli of the composite plate face sheets were 18 GPa, 16 GPa,and 5.5 GPa along direction one, direction two and direction three,respectively. Direction 1 was aligned to the warp direction, direction 2was the weft direction, and direction 3 was the transverse direction.

The hollow metallic millitubes for the grid stiffened cores were made ofstainless steel (K & S Precision Metals, Chicago, Ill.). The elasticmodulus and yielding strength of the steel tube were found to be 205 GPaand 170 MPa, respectively. Upon measuring the inner diameter and outerdiameter of the aluminum millitubes were found to be 1.92 mm and 3.15mm, respectively.

LOCTITE Hysol 9460 (Ellsworth Adhesives, Germantown, Wis.), an epoxybased structural resin was used in all three types of sandwich cores(type-I, type-II, type-III as defined herein). The density of the epoxyresin was 1.31 g/cm³. According to the manufacturer's test data, theelastic modulus, tensile strength and elongation at break were 2.76 GPa,30 MPa and 3.5%, respectively.

Specimen Preparation:

The steel millitubes were first extruded by using a mold to createequidistant indentation (every 8.0 mm), as shown in FIG. 7a . Themillitubes were assembled together, as FIG. 7b shows. After more layerswere added, the adhesive resin was then infused into the millitubespreform as shown in FIG. 7c-d , by a Resin Infusion Molding (RIM) systemfollowing conventional methodologies (see, e.g., [20]). After curing atroom temperature, the cast cores were demolded and postcured (55° C. for4 hours). The surfaces of the grid stiffened cores were ground andcleaned before applying the skins. The prefabricated laminated facesheets (E-glass/vinyl ester) were then bonded to the cores with the samepolymer resin (LOCTITE Hysol 9460) used to form the sandwich panel. Inalternative embodiments of the invention, there will generally be morethan one layer, e.g., 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15 ormore layers. Any upper limit of layers will, as understood by those ofordinary skill, depend on desired properties such as sandwich weight orthickness (e.g., a panel of the invention when used asvehicle/structural armor may be at least 10, 20, 30, 40, 50, 60 70, 80,90 or 100 cm in thickness).

The cross section of grid stiffened millitubes was shown in FIG. 8 whereit is seen that each of the millitubes was mechanically interlocked withother millitubes. Accordingly, when the sandwich panel was subjected toan impact load, the impact energy could be transferred from the localimpact point to the whole sandwich panel. Therefore, the impact energyand impact wave could be absorbed by the integrated grid stiffenedsandwich structure.

Of note, mass production of the hybrid cores can be easily realizedusing the existing equipment and procedures in the composite industry.Therefore, ease of mass production is an advantage of the sandwichpanels set forth herein.

In this example, three layers of grid stiffened steel millitubes werefabricated with 40% volume fraction of millitubes (the rest 60% was theepoxy resin) for the cores. Therefore, the grid stiffened cores had thedensity of 1.65 g/cm³.

Geometry and Test Configuration

The type-II sandwich panels were subjected to central point impact test(simply supported at two ends). The impact tests were conducted by theInstron Dynatup 8250 HV Impact testing machine with a hemi-sphere tupnose (the diameter is 12.7 mm). (Instron Corporation, Norwood, Mass.)Sandwich panels were 101.6 mm long, 25.4 mm wide and 20.5 mm thick. Theinitiation energy and propagation energy were calculated using a dataacquisition system integrated in the impact test machine.

With a 4 g bullet with 308 m/s initial velocity at impact on armor, thetotal impact energy is around 190 J. Therefore, in order to simulatesuch bullet and velocity in this study, 25 kg hammer with impactvelocity of 3.9 m/s was used for this experiment and created 190 Jimpact energy for the sandwich panels. Three specimens were tested withthis low velocity impact load (3.91 m/s).

Results and Discussion

The typical load-time and energy-time responses of the sandwich panelsunder impact loadings are shown in FIG. 9. Meanwhile, the average ofpeak load, initiation energy and propagation energy of the panels underimpact loadings are listed in Table 2.

TABLE 2 Average of impact test results of the sandwich panels ImpactInitiation Maximum velocity Energy Propagation load (kN) (m/s) (J)Energy (J) 23.5103 3.9186 48.521 143.629

Impact energy corresponding to the maximum impact force was defined asinitiation energy. Propagation energy was defined as the differencebetween the maximum impact energy and the initiation energy. Thesedefinitions had been used previously [23]. It had been suggested thatthe initiation energy was basically a measurement of the capacity forthe target to transfer energy elastically and higher initiation energyusually mean a higher load carrying capacity; on the other hand, thepropagation energy represented the energy absorbed by the target forcreating and propagating gross damage.

Interfacial Debonding:

For the type-II grid sandwich panel embodiments used in this Example, nointerfacial debonding was observed. Only some micro-scale debondingbetween the millitubes and matrix were observed after impact, while,most millitubes/matrix interfacial bondings remained intact. Withoutbeing bound by theory, it is presently understood that thegrid-configured hollow millitubes (or other configurations ofalternative structural units/elements possessing physical properties ofthe grid-configured hollow millitubes, such as a millifilament grid)provide flexibility to the sandwich panel by allowing large deformationalong with integrated mechanical interlock within the grid. Thisembodiment also had excellent impact energy absorption. Compared withother types of core, this grid stiffened core reduced local impactdamage by rapidly transferring the local impact energy to the wholestructure; further this embodiment strongly suppressed the interfacialdebonding near the face sheet. Thus, this embodiment provides a solutionto the interface debonding problem which has previously been a majorconcern in the traditional foam cored sandwich structures.

Impact Failure Mode:

As a result of the impact testing the grid-stiffened sandwich panelswere not broken into pieces, they suffered no impact windows, and werenot bent to any appreciable extent. Only an impact indentation could beseen on the top view However, a big circular area could be found on thebottom, which was an impact effect area. Comparing the diameter of theimpact effect area and the diameter of the impact indentation area, itwas found the impact effect area was 40 times that of the impactindentation area, which indicates that the impact force was transferredin a large area due to stress wave propagation throughout the gridskeleton.

Based on SEM observation it was seen that when subjected to the impactloadings, initially the impact energy and impact wave were spread ordistributed from the face sheet to the sandwich core locally, and thenpassed the first layer of grid stiffened steel millitubes and causedplastic deformation of local grid stiffened millitubes and surroundedmatrix; subsequently, the impact energy and impact wave were furtherdistributed and absorbed layer by layer due to the mechanical interlockof grid-stiffened structure; finally, the remaining weakened impactenergy and impact wave were absorbed by the bottom sheet of the sandwichpanels manifest as an impact effect area.

Thus, the grid-stiffened type-II sandwich panels manifest excellentflexibility and energy absorption ability. Based on SEM observation itwas seen that plastic deformation of the grid stiffened steel millitubesbecomes the dominant contributor of absorbing impact energy and impactwave, and most energy dissipations were assigned to the grid stiffenedmetallic millitubes and surrounded matrix. This mechanism protects thepolymer matrix from the brittle crazing even the matrix is quite brittle(with only about 3.5% maximum elongations).

Conclusions

Sandwich construction has been extensively used in various fields.However, sandwich panels have not been fully exploited in criticalstructural applications due to the concern of debonding and impactdamage. To address these problems, new grid stiffened hollow millitubecore based sandwich panels were developed as set forth herein. The testresults demonstrated: (i) the interfacial debondings at or near the facesheet/core were improved by the new grid stiffened millitube-basedsandwich panels; (ii) rather than the brittle failure which occurs inthe traditional syntactic foam core panels, significant ductile failurewas achieved in the new sandwich panels; and (iii) these new millitubegrid-stiffened sandwich panels could stand repeated impact damage. Thisdata indicated that the new sandwich panels are an option, e.g., for thecritical armor applications requiring debonding and multiple impacttolerance.

Example 3: Sandwich Panels with in-Plane Metal Tube Reinforced ShapeMemory Polymer (SMP) Foam Core

This example sets forth and characterizes a new sandwich panel whichcomprises a hybrid core with a metallic microtube-reinforced shapememory polymer matrix (rather than the polymer resin used in Example 2).Otherwise, this embodiment is generally analogous to the grid-stiffenedembodiment set forth in Example 2. In addition, here aluminum tubes wereused because: they are (1) lighter than steel tubes; and (2) toexemplify that ductile, strong yet light structural reinforcing unitsare efficacious regardless of the types of unit or tube material used.

Materials

The shape memory polymer was synthesized by using polytetramethyleneether glycol (Sigma-Aldrich, St. Louis, Mo.) as the soft segment whilediphenylmethane-4,4-diisocyanate (MDI) (Sigma-Aldrich, St. Louis, Mo.)and molecular extender 1,4-butanediol (BDO) (Acros Organics, New Jersey)as the hard segment. polytetramethylene ether glycol,diphenylmethane-4,4-diisocyanate, and 1,4-butanediol were demoisturizedbefore use. The shape memory polyurethane was synthesized bypre-polymerization technology. Remainingdiphenylmethane-4,4-diisocyanate and 1,4-butanediol (BDO) were added atlast. Finally, the reacting polymer was poured into mold covered withPolytetrafluoroethylene sheet for easy de-molding. This was done inaccordance with methodologies understood by those of ordinary skill inthe art [see, e.g., 21] The SMP was designed according to the objectivesof shape recovery temperature design.

The switching transition of the shape memory polymer was tailored byvarying the soft segment length and hard segment content. The hardsegment contents were 50.29% and 73.16%. The NCO/OH value was 1.05%.

The metallic millitubes for the hybrid cores were made of aluminum6061-T6 (K & S Precision Metals, Chicago, Ill.). The elastic modulus andyielding strength of the aluminum tubes were 72 GPa and 120 MPa,respectively. Upon measuring, it was found that the inner diameter andouter diameter of the aluminum millitube were 1.92 mm and 2.52 mm,respectively.

Specimen Preparation

The aluminum millitubes were first layered up to 3 layers in the mold,with each of the layers along the same (essentially unidirectional),longitudinal direction (thus this is an embodiment of a Type-2 panelwhere all layers are unidirectional and now SMP is the filler). Afterall layers were added, the shape memory polymer was then infused intothe millitubes preform by a Resin Infusion Molding (RIM) systemaccording to conventional methodologies (see, e.g., [20]). After curingat 100° C. temperature for 12 hours, the cast cores were demolded andpostcured (70° C. for 4 hours). The material and method of conventionalSMP sandwich core was used the same material as used for the aluminummillitubes reinforced SMP.

As mentioned in Example 2, mass production of the hybrid cores can beeasily realized using existing equipment and procedures in the compositeindustry. Therefore, ease of mass production is an advantage of sandwichpanels in accordance with the invention, as set forth herein.

In this study, three layers of millitubes were fabricated with 40%volume fraction of millitubes (the rest 60% was the epoxy resin) for thecores. Therefore, the hybrid cores had the density of 1.1 g/cm³. Theglass transition temperature is 38° C.

Geometry and Test Configuration

The sandwich panels were subjected to central point impact test (simplysupported at two ends). The impact tests were conducted by the InstronDynatup 8250 HV Impact testing machine with a hemi-sphere tup nose (thediameter is 9 mm). Sandwich core were 101.6 mm long, 101.6 mm wide and20.5 mm thick. The initiation energy and propagation energy werecalculated using a data acquisition system integrated in the impact testmachine, following the analogous protocol set forth in Example 2.

In order to simulate a 4 g bullet with 308 m/s initial velocity andimpact on armor, the total impact energy is around 190 J. Therefore, inthis study, 25 kg hammer with impact velocity of 3.9 m/s was used forthis experiment and could create 190 J impact energy for the sandwichpanels, following the analogous protocol set forth in Example 2.

Results and Discussion

Five specimens were tested in the specified impact velocity (3.91 m/s)protocol. The typical load-time and energy-time responses of thesandwich core under impact loadings are shown in FIG. 10. The average ofpeak load, initiation energy and propagation energy of the panels underimpact loadings were calculated. The peak load, initiation energy andpropagation energy of the sandwich core under impact loadings were 25KN,45 J, and 147 J respectively.

As in Example 1 and Example 2, impact energy corresponding to themaximum impact force is defined as initiation energy. Propagation energyis defined as the difference between the maximum impact energy and theinitiation energy.

Interfacial Debonding:

No undesired interfacial debonding of the face sheet from the sandwichcore was observed under impact loading conditions. Only some micro-scaledebondings between the millitubes and matrix were observed, while mostmillitube/matrix interfacial bondings remained intact. A significantductile deformation and failure mode were observed. Without being boundby theory, it is believed that the horizontally aligned hollowmillitubes provide the flexibility to the sandwich panel by allowinglarge deformation and plastic hinges within the millitubes.

Impact Failure Mode:

When the sandwich panels were subjected to impact loading, a largeimpact hole was created in the pure shape memory polymer core and asmall impact indentation was observed in the SMP-millitubes sandwichcore. When subjected to the impact loadings, the type-II sandwich coresof this example were found to have excellent flexibility and energyabsorption ability. The plastic deformation of the aluminum millitubesbecame the dominant contributor to impact energy absorption, and mostenergy dissipations are assigned to the metallic millitubes. Thismechanism protects the polymer matrix from the brittle crazing eventhough the matrix is relative brittle. As noted, almost no interfacedebonding occurred in the tested specimens. Accordingly, this embodimentprovides a solution to the interface debonding problem which has been amajor concern in the traditional foam cored sandwich structures.

Conclusions

The test results demonstrated: (i) interfacial debondings at or near theface sheet/core were largely excluded from these type-II sandwich cores(comprising horizontally aligned aluminum millitubes); (ii) thesignificant ductile failure was achieved in the tested panels ratherthan the brittle failure that occurred in panels with traditional SMPcores. The data in this Example indicated that the sandwich core ofaluminum millitube grid-reinforced SMP is an option for criticalstructural applications requiring debonding and impact tolerance.Moreover, in accordance with the invention, it was shown thatembodiments with SMP may be used to advantage because of the ductilityof this polymer.

Example 4: Impact Mitigation of Sandwich Panels with Metal (Steel) TubeGrid as Reinforcement of a Foam Core

Materials and specimen preparation was performed as set forth in Example2 with the exception that the steel millitubes were extruded by using amold to create equidistant indentation (every 12.70 mm). In thisExample, three types of grid stiffened millitubes were fabricated with40% volume fraction of steel millitubes (the rest 60% was the epoxyresin) for each core regardless of the number of layers of the millitubegrid skeleton. The specimens were divided into three groups G1, G2, andG3 which had 1 layer, 2 layers, and 3 layers of millitube grid skeleton,respectively. All of these (G-1, G-2, G-3) are “type-II panels” asdefined in Example 1. The density of the sandwich panel of G1, G2, andG3 were 1.62, 1.78, and 1.95 g/cm³, respectively.

Geometry and Test Matrix

The sandwich panels were prepared (i.e., the face sheets were added)after 24 hours curing at room temperature and 1 hour post cure at 100°C.

Sandwich panels were subjected to two types of static tests: (i) staticcompression tests; (ii) static three-point bending test (simplysupported at two ends). In addition the sandwich panels were subjectedto two types of impact tests: (i) low velocity impact test as set forthin Examples 1 and 2; and (ii) ballistic impact tests with bullets firedfrom various types of guns.

Using the analogous protocol as set forth in Example 1, in the axialcompression tests and three-point bending tests, the strain controlledloading mode was employed using MTS 810 machine (MTS SystemsCorporation, Eden Prairie, Minn.) with a loading rate of 0.6 mm/min. Thespecimen sizes of G1, G2, and G3 for the bending test are 101.6 mm long,25.4 mm wide, and 6.2 mm, 12.4 mm, and 18.6 mm high, respectively. Thespecimen sizes of G1, G2, and G3 for the compression test are 25.4 mmlong, 25.4 mm wide, and 6.2 mm, 12.4 mm, and 18.6 mm high, respectively.For the three-point bending test, the span length was 90 mm.

Using the analogous protocol as set forth in Example 1, sandwich panelswere also subjected to low velocity impact test. The impact tests wereconducted by the Instron Dynatup 8250 HV Impact testing machine (InstronCorporation, Norwood, Mass.) with a hemi-sphere tup nose (the diameterwas 12.7 mm). Dimensions of the G1, G2, and G3 sandwich panels are 101.6mm long, 50.8 mm wide, and 12.7 mm, 19.1 mm, and 25.2 mm high,respectively. The initiation energy and propagation energy werecalculated using a data acquisition system integrated in the impact testmachine. For a 4 g bullet with 308 m/s initial velocity impact on armor,the total impact energy is around 190 J. To simulate this situation, inthis study a 25 kg hammer with impact velocity of 3.9 m/s was used whichcreated 190 J impact energy for the sandwich panels.

Sandwich panels were also subjected to ballistic impact testing. Theseimpact tests were conducted by an XD pistol (Springfield Armory,Geneseo, Ill.) with 9 mm bullets, and a Ruger 10/22 revolver (Sturm,Ruger & Co., Inc., Newport, N.H.) with .22 caliber hollow point bullets.Dimensions of the sandwich panels were the same as the specimens in lowvelocity impact test. The high velocity impact test setup is shown inFIG. 11. The bullet speed was measured with laser speedometer. Theshooting angle was 90° to the target.

Results and Discussion

Axial Compression Tests

The typical compressive stress—strain curves of the three types ofsandwich cores are plotted in FIG. 12. It is observed that G1, G2, andG3 have almost the same compression strength 68 MPa at 30% strain.However, as the number of steel grid layers increased, the modulus ofthe sandwich structure is increased with 281.56 MPa, 390.22 MPa, and505.62 MPa for G1, G2, and G3, respectively. It was also observed thatthe compression stress of the three types of sandwich cores convergeafter 30% strain. This may be due to the consolidation of the hollowtubes. Once the hollow tubes were consolidated, the difference among thethree groups was negligible. It was observed that upon the hollow steeltubes being squeezed, cracks propagated around the boundaries of thesteel tubes due to interfacial shear stress.

Three-Point Bending Tests

Typical bending stress-strain curves for the three types of sandwichcores are plotted in FIG. 13. The flexural strength of G-1, G-2, and G-3were 36.1 MPa, 50.2 MPa, and 62.5 MPa, respectively. It is observed thatthe maximum bending strain and bending strength were increased as thenumber of the grid stiffened layers increased. It was also observed thatas the number of the grid stiffened layer increased, the stress-straincurve behaved similarly to the post-yielding plastic deformation ofmetal materials due to the geometry of the hollow metal millitubes. Theload first caused elastic deformation of the millitubes, followed byconsolidation of the millitubes due to squeezing out of the air,yielding and plastic deformation of the consolidated millitubes. Thecrack initiation and propagation were detected along the loading lineand from the bottom surface of the sandwich core to the middle plane.The rest of the sandwich core was not damaged. The crack propagation wasalong the interface between the adhesive and the hollow tubes, believedto be due to the stress concentration at the polymer/tube interface.

Quasi-Static Low Velocity Impact Tests

Impact Response

Low velocity impact test was conducted at three locations (bay, node,and rib) of the sandwich panel. The typical load-time and energy-timeresponses of the G-3 sandwich panels under impact loadings are shown inFIG. 14(a-c). The bay area was 10 mm×10 mm, the hammer impact area was a12 mm diameter rounded area.

Impact energy corresponding to the maximum impact force was defined asinitiation energy. Propagation energy was defined as the differencebetween the maximum impact energy and the initiation energy. Thesedefinitions have been used previously [19]. It has been suggested thatthe initiation energy is basically a measurement of the capacity for thetarget to transfer energy elastically and higher initiation energyusually means a higher load carrying capacity; on the other hand, thepropagation energy represents the energy absorbed by the target forcreating and propagating gross damage.

The peak load and propagation energy of the panels with different impactlocations are presented in FIG. 15(a) and FIG. 15(b). One can see thatthe node areas have a higher load carrying capacity and higherpropagation energy as compared with bay areas and rib areas of eachgroup. As the number of the millitube layers increased, the loadcarrying capacity and propagation energy increased at the same impactlocation of each group.

In FIG. 14(a-c), it is seen that the shapes of the load-time curves weredifferent as the impact location changes. When the tup impacted at a bayarea, the impact energy and impact wave were constrained by thesurrounding steel tube-stiffened grid skeleton, with energy thentransferred to the entire sandwich structure by the interlockedmillitube and surrounded adhesive matrix. Therefore, the major energytransfer process was dependent on the adhesive matrix. However, when thetup impacted at the node area, the impact energy and impact wave weredirectly transferred by the millitube stiffened grid and interlockingmechanism. The impact wave was distributed at each node and transferredalong two directions (the grid skeleton) and absorbed by the entiresandwich structure. Therefore, the load-time curve of the node areaexhibited damping characteristic, as shown in FIG. 14(b). Thepropagation energy of the node area was 148.65 J, the highest ascompared with a bay area (107.48 J) or rib area (128.67 J). For impactat a rib, the transfer of impact energy and impact wave was between thatseen with the impact at a bay and impact at a node because the stresswave propagates mainly along one direction in the grid skeleton.

Failure Mode

For the sandwich panels under low velocity impact, no interfacialdebonding was observed. Only some micro-scale debondings between themillitubes and matrix were observed after impact, while mostmillitubes/matrix interfacial bondings were still in contact. Withoutbeing bound by theory, it is believed that the millitubes (in a gridconfiguration further supported by tube nodes) provide flexibility tothe sandwich panel by allowing large deformation while maintainingintegrated mechanical interlock within the millitubes in the grid. Thisembodiment had excellent impact energy absorption and strong suppressionof interfacial debonding near the face sheet. Accordingly, the inventionprovides a solution to the interface debonding problem which has been amajor concern with the traditional foam-cored sandwich structures.

It was also observed that the grid-stiffened sandwich panels were notbroken into pieces, there were no impact windows “Impact window” refersto the open space created in the face sheet and core, whereas “impactindentation” refers to plastic deformation on the face sheet, and theyeven fully rebounded after impact. Only an impact indentation could beseen on the top view. Accordingly, it was found that this new sandwichpanel could withstand more than a one-time impact loading. A bigcircular area could be found on the bottom, which was impact effectarea. After measuring the diameter of impact effect area and impactindentation area, it is found that the impact effect area was 40 timesof impact indentation area. This phenomenon is consistent with thepresently understood impact energy transfer mechanism.

Based upon observation, it was found that when the sandwich wassubjected to the impact loadings, initially, the impact energy andimpact wave were spread or distributed from the face sheet to thesandwich core locally and then passed the first layer of steelmillitubes stiffened grid and caused plastic deformation of localmillitubes and surrounding matrix. The impact energy was distributed asstress wave propagated in the sandwich structure. The stress wave waspropagated along the orthotropic millitube grid direction and theisotropic adhesive matrix. It was further propagated to the layersbeneath and caused new deformation of millitube and matrix at eachmillitube grid node area due to the mechanical interlocking. It wasobserved that crack propagation was along multiple tracks instead of asingle track in the sandwich core. This phenomenon also explained whythe load-time curve presented damping characteristic in FIG. 14(b).Finally, the remaining small amount of impact energy and impact wave wasabsorbed by the bottom sheet of sandwich panels. The whole impact damagezone was cone shaped.

Thus, these grid stiffened sandwich panels possessed excellentflexibility and energy absorption ability. The plastic deformation ofthe steel millitube-stiffened grid becomes the dominant contributor ofabsorbing impact energy and impact wave, and most energy dissipationswere assigned to the grid skeleton and surrounding matrix. Thismechanism protects the polymer matrix from the brittle crazing eventhough the matrix is quite brittle (with only about 3.5% maximumelongations).

Ballistic Impact Test

Three types of sandwich panels (G1, G2, G3) were subjected to thisballistic impact testing. The shooting distance between target andpistol was 1 M. The bay area of the sandwich panel was chosen as idealimpact location. However, actual impact locations were distributed onthe bay, rib, and node areas, due to the recoil of the pistol.

Two types of bullets (9 mm and 0.22 hollow point) were used asprojectiles. The bullet weights were 7.45 g and 2 g, respectively. Themuzzle velocities were 390 m/s and 440 m/s, respectively. The impactenergy was 560 J and 190 J, respectively. For the single layer millitubegrid sandwich panel, the 9 mm bullet penetrated the face sheet, but wascaught by the grid skeleton within the panel, as shown in FIG. 16. FIG.16(a) shows the top view of the FRP face sheet after impact. In FIG.16(b), the black lines indicate the location of the millitubes. It isobserved that the bullet was caught at the corner of the bay area. Thebullet has a 9 mm diameter and still was caught by the 12.7×12.7 gridskeleton. FIG. 17 presents the 9 mm bullet deformation before and afterballistic impact test on the two layer grid millitube cored sandwichpanel.

For two layers (and it is expected for three layers) millitube gridskeleton cored sandwich panel, the 9 mm bullet could not perforate thewhole panel and was caught by the grid skeleton at the bottom of thepanel, as shown in FIGS. 17(a) and (b) which show the top view andbottom view of the sandwich panel after impact, respectively. It wasobserved that the bullet was also caught at the corner of the bay area.The impact effect area was almost the same as that in FIG. 16(b). Thisresult further validates the hypothesized impact energy transfermechanism set forth above, even in the ballistic impact condition.

Some G2 sandwich panels were tested by using .22 caliber hollow pointbullet (5.6 mm diameter), as shown in FIG. 18. A 0.22 bullet only has 2g weight but its muzzle velocity is 440 m/s and impact energy is 190 J.The bullet diameter is 5.6 mm which is very small compared with the 12.7mm×12.7 mm bay size. After the 0.22 bullet impacted the sandwich panel,it was crushed and fragmented to several pieces. However, the shellfragments were still caught by the adhesive matrix. This resultindicates that the a sandwich panel of the invention can catch a bulleteven if the bullet caliber is much smaller than the bay size.

In order to test the multiple impact resistant ability of sandwichpanels in accordance with the invention, G2 specimens were shot twice byusing both a 9 mm bullet and a .22 caliber bullet, as shown in FIG. 19.FIGS. 19(a) and (b) show the top view and the bottom view of thesandwich core, respectively. It is observed that the two bullets wereconfined by the millitube grid skeleton cored sandwich panel. The impactenergy was absorbed or transferred by the plastic deformation of themillitubes and fracture of the adhesive. Neither bullet perforated thesandwich panel, and each stayed in the crater it created. It wasobserved that 9 mm and .22 caliber bullets are all deformed and squeezedwith 560 J and 190 J impact energy. These results indicate that thesandwich panel in accordance with the invention can withstand more thana single instance of a ballistic impact load, such as may occur in amilitary conflict or other attack situations.

Various commercially available bullet-proof and blast-resistant panelsare currently used in many engineering applications, such asbullet-proof walls, glass doors, and so on. However, their weight andcosts can be high. Moreover they require high temperature compressionfabrication, whereas in the present invention one can fabricate at roomtemperature. Also, commercially available bullet-proof andblast-resistant panels only have glass fiber reinforced polymer laminatecomposites. For example, we compared certain commercial products, andTable 3 summarizes the cost and density of panel embodiments G1, G2, G3of the present invention with some commercially available productsGlasticShield™ (GlasticShield™, Cleveland, Ohio) and Armortex®(Armortex, Schertz, Tex.). It was observed that the proposed sandwichpanel has the lowest density and very economical cost.

TABLE 3 Cost and density comparison of G1, G2, G3 panel embodiments withselected commercially available products Density Cost Sample Types(g/cm³) ($/cm²) GlasticShield ™ 1.99 data not available Armortex^(R)2.43 0.013 G1 1.62 0.0073 G2 1.78 0.0091 G3 1.95 0.011

The ballistic impact performance of G1, G2 and G3 embodiments was alsocompared with the same two commercial products—GlasticShield™ andArmortex®. The results are presented in Table 4. As compared with the 9mm and .22 caliber bullets, the proposed sandwich panel can stop thebullet, as shown in FIGS. 16-19, and with lower weight and low cost.Therefore, panels of the invention are understood to have similarperformance as compared to various state-of-the-art products, yet havingthe advantages of lower weight and low cost.

TABLE 4 Performance Comparison between of G1, G2, G3 panel embodimentswith selected commercially available products Sample Types BulletVelocity (m/s) GlasticShield ™ 0.44 411 Armortex^(R) 9 mm 358 G1 9mm/.22 390/440 G2 9 mm/.22 390/440 G3 9 mm/.22 390/440

Conclusions

The test results demonstrated that: (i) interfacial debondings at ornear the face sheet/core were eliminated or significantly reduced; (ii)rather than the brittle failure which occurs in the traditionalsyntactic foam cores, significant ductile failure was achieved with thesandwich panels of the invention; (iii) sandwich panels in accordancewith the invention can be of light weight and may be used as bulletproofarmor; (iv) sandwich panels of the invention could withstand multipleballistic impacts in the same general region; and, (v) the panels of theinvention had lower weight ratio, lower cost, and better impactresistance as compared to various commercially available products. Thisdata shows that sandwich panels in accordance with the invention are anoption for critical armor applications which require debondingresistance and multiple impact tolerance.

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All documents cited herein, whether patent or non-patent literature, andall patent-related documents relied on for priority or relatedness forthis application are fully incorporated by reference herein for allpurposes.

What is claimed:
 1. A composite material comprising a matrix and a grid;wherein: (a) said matrix comprises polymer; (b) said grid comprises aplurality of layers, wherein each said layer comprises a network ofmechanically interlocking millitubes or microtubes; (c) said polymer hasan elongation at break from 3% to 200%; an elastic modulus from 1.5 GPato 350 GPa; and an ultimate tensile strength from 25 MPa to 350 MPa; (d)said millitubes or microtubes comprise hollow, shape memory-alloy tubes,with a diameter of 3 mm or less; an elongation at break from 3% to 100%;an elastic modulus from 1 GPa to 1000 GPa; and an ultimate tensilestrength from 1 MPa to 1000 MPa; (e) i) said layers that are adjacent toone another are interwoven with one another; or ii) said adjacent layerscomprise indentations that are interposed or seated within one another,or both; or both i) and ii); (f) said matrix surrounds and providessupport to said grid, and said grid confines and increases the strengthof said matrix.
 2. The composite material of claim 1, wherein saidmillitubes or microtubes in adjacent said layers are parallel to oneanother.
 3. The composite material of claim 1, wherein said millitubesor microtubes in adjacent said layers are perpendicular to one another.4. The composite material of claim 1, wherein said millitubes ormicrotubes in adjacent said layers are interwoven with one another. 5.The composite material of claim 1, additionally comprising a face sheetbonded to at least one surface of said material; wherein said face sheetcomprises a laminated composite comprising polymer reinforced with glassfibers, carbon fibers, or both.
 6. The composite material of claim 1,additionally comprising a face sheet bonded to each of two oppositesurfaces of said material; wherein each of said face sheets comprises alaminated composite comprising polymer reinforced with glass fibers,carbon fibers, or both.
 7. The composite material of claim 1, whereinsaid millitubes or microtubes in adjacent said layers are aligned at anangle between 0° and 90° with respect to one another.